1. Field of the Invention
The present invention relates to a common rail fuel injection control device suitable for diesel engines, more specifically to a method for controlling a metering valve for adjusting the quantity of fuel pumped into the common rail.
2. Description of the Related Art
In common rail fuel injection control devices for diesel engines, high-pressure fuel with a pressure increased to an injection pressure (for example, from several tens to several hundreds of MPa) is accumulated in a common rail at a pressure, and this fuel is injected into cylinders by opening the valves of injectors. As for fuel supply into the common rail, fuel pumping is conducted with a supply pump serving as a high-pressure pump, and the quantity of fuel flowing into the supply pump is adjusted with a metering valve. The opening degree of the metering valve is controlled according to the drive signal supplied from a controller, and the quantity of supplied fuel is thus controlled. As a result, the common rail pressure is controlled. The metering valve is composed, for example, of an electromagnetic valve of a spool valve type.
A process for controlling the quantity of fuel supplied to the supply pump and thereby controlling the quantity of fuel pumped by the supply pump and controlling the common rail pressure has already been known (for example, Japanese Patent Applications Laid-open No. H11-30150 and S63-50469).
However, the problem associated with such a common rail fuel injection system was that valve sticking occurred when the engine operation state (for example, idling) with a constant opening degree of the metering valve was maintained. In other words, because an action overcoming a static friction force is required to move the valve from a state in which it was stopped in a fixed position, comparatively large changes in electric current have to be induced. Furthermore, if the state with a constant valve opening degree is maintained for a certain time, lubrication in the sliding parts of the valve is further degraded and the trend to valve sticking is further increased (the static friction force increases). As a result, the responsiveness of the valve to current changes is degraded.
This will be explained with reference to FIG. 6. In the figure an electric current fed to the metering valve is plotted against the abscissa, and the opening degree of the metering valve is plotted against the ordinate.
The figure shows that, for example, an electric current i2 is required (point I) to open the metering valve from a completely closed state to an opening degree V. When the valve opening degree remains constant for a comparatively long time in this state, a comparatively large current change Δi becomes required to actuate the metering valve thereafter in the closing direction. In other words, the valve starts moving in the closing direction from the point in time (point II) in which the electric current fed to the metering valve is decreased by Δi from i2 to i1. Therefore, the interval with current changes Δi becomes a non-sensitive zone in which the valve opening degree does not change in response to changes in the electric current.
When the non-sensitive zone caused by valve sticking thus occurs, even if the electric current value is changed with the object of causing transitional changes in the common rail pressure, the responsiveness of the metering valve to changes in the electric current valve is poor. As a result, inadequate tracing of common rail pressure occurs.